Limiting radial preloads in securing an orthopedic fastener

ABSTRACT

An orthopedic fastener rated to be received into a corresponding bone specimen. The corresponding bone specimen has (1) an overall outer thickness from near cortex to far cortex in a predetermined rated range of outer bone thicknesses, and (2) a predetermined bone hole preparation including an entry portion in the near cortex. The entry portion has a predetermined minimum entry diameter. The fastener provides a tapered portion. When the fastener is operably secured into the far cortex of a bone thickness for which it is rated to be received, the tapered portion of the fastener contacts the entry portion in the near cortex so as to impart no more than about 0.2 mm of radial preload into the bone surrounding the entry portion.

RELATED APPLICATIONS

This application is related to commonly-invented, concurrently-filedU.S. patent application “Engaging Predetermined Radial Preloads inSecuring an Orthopedic Fastener”, Ser. No. 12/592248, the disclosure ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to orthopedic fasteners, and morespecifically, in preferred embodiments, to orthopedic fastenersconfigured to exert, when secured into cortical bone, no more than apredetermined amount of radial preload in the cortical bone materialsurrounding the fastener's regions of contact with the bone.

BACKGROUND OF THE INVENTION

A common complication of fracture repair, external fixation (or otherorthopedic procedures in which fasteners are inserted into bone) isloosening of the fastener over time at the fastener/bone interface. Intheir article “The Effect of Radial Preload on the Implant/BoneInterface: A Cadaveric Study”, Biliouris et al. suggest several reasonsfor this loosening, including micro-movement of the bone tissue as axialor bending loads are exerted on the fastener during normal use of thefastened bone by the patient after surgery. As described in some detailin the above-referenced article (hereafter referred to as “Biliouris etal.”), such axial and bending loads can cause circumferential and othermicro-displacement of the fastener as secured into the bone. Thismicro-displacement temporarily deforms the shape and size of the bonehole into which the fastener is secured, resulting in loss ofcircumferential contact between the fastener and the bone, which in turnleads to higher stress conditions at the remaining areas of contact.However, while the higher elasticity of the metal fastener allows thefastener to resume its original shape and relative position within thebone hole after loading, the lower elasticity of the bone surroundingthe bone hole leaves the hole potentially damaged after loading. As aresult, there is a loss of contact between fastener and the bone duringand potentially after loading, resulting in micro-motion betweenfastener and bone, manifesting itself over time and repetitive loadingas loosening of the fastener.

Biliouris et al. also discuss the use of radial preload to counteractloosening of fasteners under repetitive loads in external fixation. Thebasic concept of radial preload is to “oversize” the diameter of thefastener within the bone hole. In this way, the bone material around the“oversized” portion of the fastener is compressed, tightening thecontact between the fastener and the bone material, encouraging intimateand sustained contact during operational loading, and thereby reducingthe propensity for loosening.

However, as taught by Biliouris et al., the bone material'scomparatively low elasticity limits the amount of radial preload thatcan be absorbed by the bone material without causing cracking of thebone material around the oversized portion of the fastener. Biliouris etal. observe that a fastener diameter more than about 0.2 mm greater thanthe receiving bone hole is prone to cause micro-cracking in surroundingcortical bone material.

The reference in this disclosure so far to “bone” deserves furtherdiscussion. Orthopedic fasteners are typically designed to fasten eithercortical bone or cancellous bone. Cortical bone, typically found on theoutside of a bone, is much tougher and harder than cancellous bone,which is typically found on the inside of a bone. Cancellous bone hassoft and malleable characteristics, whereas cortical bone isconsiderably harder. While radial preload is understood to provideadvantages in both cortical and cancellous bone, the inventive focus ofthis disclosure is on the radial preload imparted by fasteners securedin cortical bone.

U.S. Pat. No. 6,949,100 (Venturini) discloses an orthopedic fixation pinwith a tapered thread. While Venturini mentions use in the art of radialpreload, Venturini's disclosure and invention focuses on thread profilegeometry as a way to enhance the fixation pin's grip on bone.

U.S. Pat. No. 5,961,524 (Crombie) discloses an orthopedic fastener witha tapered thread. The fastener is configured to be received into asmooth hole of substantially the same taper as the thread on thefastener. Once secure into the into the hole, the fastener is given asmall amount of extra tightening to compress bone material surroundingthe threads in order to improve grip of the fastener on the bone.Crombie's disclosure, however, appears to be solving the problem ofimproving bone-to-fastener contact rather than imparting limited orcontrolled amounts of radial preload into the bone surrounding thefastener. As a result, no structure to measure or limit radial preloadis disclosed.

U.S. Pat. No. 7,198,488 (Lang et al.), U.S. Pat. No. 5,593,410 (Vrespa)and U.S. Pat. No. 6,953,463 (West, Jr.) illustrate fastener threadstyles understood to be advantageous in improving fastener grip ineither cortical or cancellous bone. These disclosures provide noguidance, however, on imparting limited or controlled amounts of radialpreload into the bone surrounding the fastener.

Likewise U.S. Pat. No. 6.565,573 (Ferrante et al.) and U.S. Pat. No.6,375,657 (Doubler et al.) disclose bone fasteners with generallytapered threads, but the disclosures of these patents provide noguidance on imparting limited or controlled amounts of radial preloadinto the bone surrounding the fastener.

U.S. Pat. No. 7,001,389 (Navarro et al.) discloses fasteners withvarious thread configurations, including tapered threads, to assist insecuring a plate to bone. As with other prior art discussed above,however, Navarro et al. provide no guidance on imparting limited orcontrolled amounts of radial preload into the bone surrounding thefastener.

There is therefore a need in the art for an orthopedic fastenerspecially designed to exert radial preload on the surrounding bone sothat the radial preload reduces the likelihood of loosening of thefastener under repetitive loads. The diameter of such a new fastenerwill advantageously not exceed the diameter of the bone hole by morethan about 0.2 mm along the portions where radial preload is beingimparted. In this way, the amount of radial preload imparted into thebone will be limited to no more than about 0.2 mm. Such a new fastenermay further, or alternatively, advantageously provide structure, inother embodiments, to engage a predetermined amount of radial preloadfor a particular installation.

SUMMARY OF THE INVENTION

The present invention addresses one or more of the above-described needsidentified in the prior art. One aspect of this invention includes anorthopedic fastener rated to be received into a corresponding bonespecimen. The corresponding bone specimen has (1) an overall outerthickness from near cortex to far cortex in a predetermined rated rangeof outer bone thicknesses, and (2) a predetermined bone hole preparationincluding an entry portion in the near cortex. The entry portion has apredetermined minimum entry diameter.

The fastener itself comprises a shank providing adjacent threaded andunthreaded portions. The unthreaded portion provides a tapered diameterportion, the tapered diameter portion transitioning from a large taperdiameter to a small taper diameter. The small taper diameter is locatedproximate to the threaded portion, while the large taper diameter islocated distal from the threaded portion.

The small taper diameter is not greater than the predetermined minimumentry diameter in the bone hole preparation into which the fastener israted to be received. The large taper diameter not greater than about0.2 mm more than said predetermined minimum entry diameter. The tapereddiameter portion is located on the shank such that, when the threadedportion on the fastener is secured into the far cortex on a bonespecimen having an outer bone thickness in the predetermined ratedrange, the tapered diameter portion of the fastener contacts the entryportion in the near cortex.

It will be appreciated from the above summary that fasteners of thepresent invention are rated for particular predetermined bone specimensinto which they are to be received and secured. This “rating” aspect ofthe fasteners is discussed in more detail near the end of the “DetailedDescription” section of this disclosure.

It is therefore a technical advantage of the invention to provide anorthopedic fastener that, once secured in a specimen of bone, is lessprone to loosening. Assuming that the fastener of the present inventionis secured into a bone specimen for which the fastener is rated, thetapered diameter portion on the shank will reside in entry portion inthe near cortex when the threaded portion on the fastener is securedinto the far cortex.

Now resident in the entry portion, the tapered portion effectivelyprovides an “oversized” portion of the shank operative in the entryportion, in that the tapered diameter portion provides a large taperdiameter that is not more than about 0.2 mm greater than thepredetermined minimum entry diameter in the entry portion. By beinggreater than the minimum entry diameter in the entry portion, the largetaper diameter therefore exerts radial preload in the entry portion.However, by capping the large taper diameter at about 0.2 mm greaterthan the minimum entry diameter in the entry portion, the maximum amountof radial preload that can actually be exerted is limited to about 0.2mm, an amount recognized in the art to be unlikely to cause damage tothe cortical bone material surrounding the entry portion. In this way,radial preloads up to about 0.2 mm can be exerted in order to controlmicro-motion, subject to a limit of about 0.2 mm in order to avoid bonedamage.

A further technical advantage of the invention is to provide fastenersthat are relatively simple to manufacture, and that can be rated to bereceived into a variety of bone hole preparations in a variety of bonespecimen types and thicknesses.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiment disclosed may be readily utilized as a basisfor modifying or designing other structures for carrying out the samepurposes of the present invention. It should be also be realized bythose skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1A, 1B and 1C illustrate, according to a first aspect of thepresent disclosure, a deployment of fastener 100 into bone specimen 120through opening 135;

FIGS. 2A, 2B and 2C illustrate, according to a second aspect of thepresent disclosure, a deployment of fastener 200 into bone specimen 220through opening 235;

FIGS. 3A, 3B and 3C illustrate, according to a third aspect of thepresent disclosure, a deployment of fastener 300 into bone specimen 320through opening 335; and

FIGS. 4A, 4B and 4C illustrate exemplary embodiments of structure thatmay be deployed on fasteners of the present invention to limit themaximum radial preload available to be imparted in “mating taperedthread” configurations described herein.

DETAILED DESCRIPTION

FIGS. 1A, 1B and 1C should be viewed together, and depict a first aspectof the present disclosure, in which an orthopedic fastener 100 isconfigured to limit and/or control radial preloads exerted in nearcortex 125 as fastener 100 is operably secured into bone specimen 120.FIG. 1A illustrates a segment of fastener 100 in isolation. FIG. 1Billustrates bone specimen 120 with opening 135 prepared to receivefastener 100. FIG. 1C illustrates fastener 100 received into opening 135and operably secured into bone specimen 120.

Referring first to FIG. 1A, fastener 100 provides threaded portion 102on one end. Fastener 100 further provides unthreaded cylindrical portion103 with a pre-selected constant diameter D_(F). Threaded portion 102and cylindrical portion 103 are separated by a smooth tapered portion115. Tapered portion 115 is bounded by reference points 105 and 110, asillustrated on FIG. 1A. Reference point 105 is the point at whichcylindrical portion 103 ends and tapered portion 115 begins. Referencepoint 110 is the point at which tapered portion 115 ends. Taperedportion 115 reduces the diameter of fastener 100 from D_(F), atreference point 105, to a diameter less than D_(F) at reference point110. It will be further appreciated that, as described in more detailbelow with respect to FIG. 1C, the distance on fastener 100 betweenreference points 105 and 110 is pre-selected.

In the embodiment of fastener 100 depicted in FIG. 1A, threaded portion102 begins at reference point 110. The invention is not limited in thisregard, however. It will be appreciated that in other embodiments (notillustrated), an unthreaded portion may be provided between referencepoint 110 and threaded portion 102.

Turning now to FIG. 1B, bone specimen 120 is illustrated incross-section, showing near and far cortices 125, 126 surroundinginterior portion 130. It will be appreciated that interior portion 130may comprise cancellous bone and/or fatty bone marrow. Bone specimen 120as illustrated on FIG. 1B has been prepared to receive fastener 100 fromFIG. 1A. More specifically, opening 135 has been pre-configured toreceive fastener 100. Circular entry hole 140 has pre-selected diameterD_(O), and extends through near cortex 125 into interior portion 130 ofbone specimen 120. In the embodiment illustrated in FIG. 1B, opening 135further provides female threaded section 145 provided through to farcortex 126. The threads on female threaded section 145 are pre-selectedto mate with threaded portion 102 on fastener 100 (as shown on FIG. 1A).It will be appreciated that, although not illustrated on FIG. 1B,alternative embodiments will omit female threaded section 145, orprovide a small diameter pilot hole in its place suitable to receiveself-tapping threads deployed on threaded portion 102. In suchalternative embodiments, threaded portion 102 on fastener 100 (as shownon FIG. 1A) will provide self-tapping threads. The inventive materialdisclosed herein is independent of the manufacturing technique used toprovide female threaded section 145 in opening 135.

FIG. 1C shows fastener 100 (as illustrated in isolation on FIG. 1A)received into opening 135 in bone specimen 120 (as illustrated inisolation on FIG. 1B). FIG. 1C shows threaded portion 102 on fastener100 operably secured into threaded section 145 in opening 135. D_(F) onfastener 100 is pre-selected to be no more than about 0.2 mm greaterthan D_(O) at entry hole 140 in near cortex 125. The length of taperedportion 115 on fastener 100 (as shown on FIG. 1A) is pre-selected toenable fastener 100 (as shown in FIG. 1C) to contact entry hole 140along the length of tapered portion 115 (shown on FIG. 1A). In this way,as fastener 100 on FIG. 1C is drawn into opening 135 by rotatingthreaded portion. 102 into threaded section 145, reference point 105 ismoved closer to entry hole 140, urging D_(O) on entry hole 140 toenlarge. Accordingly, radial preload is exerted at the interface ofentry hole 140 and fastener 100, allowing the advantages of radialpreload to benefit the securement of fastener 100 in bone specimen 120.However, in accordance with the invention, as fastener 100 is drawnfurther into opening 135, and as reference point 105 passes into entryhole 140, since D_(F) is pre-selected to be no more than about 0.2 mmgreater than D_(O), the maximum amount of radial preload that securementof fastener 100 in bone specimen 120 can exert on entry hole 140 isabout 0.2 mm. This value is consistent with the teaching of Biliouris etal., discussed in the background section of this disclosure, findingthat about 0.2 mm is generally a maximum amount of radial preload that aspecimen of cortical bone can withstand without suffering damage such asmicro-cracking.

With further reference to FIGS. 1B and 1C, it will be appreciated that,although not illustrated, other embodiments may provide opening 135 witha profile through near cortex 125 that is shaped other than acylindrical entry hole such as item 140, as illustrated. For example,the profile of opening 135 through near cortex 125 could be tapered. Insuch alternative embodiments, it will be understood that D_(O), asillustrated on FIG. 1B, will be provided as a predetermined minimumentry diameter constricting the profile of opening 135 through nearcortex 125.

Turning now to FIGS. 2A, 2B and 2C, which should be viewed together, asecond aspect of the present disclosure is illustrated, in which anorthopedic fastener 200 is configured to limit and/or control radialpreloads exerted in far cortex 226 as fastener 200 is operably securedinto bone specimen 220.

Referring first to FIG. 2A, orthopedic fastener 200 provides taperedportion 215 on one end 201 of fastener 200. Fastener 200 furtherprovides unthreaded cylindrical portion 203 adjacent to tapered portion215. Cylindrical portion. 203 is of constant diameter D_(C). Referencepoint 205 on fastener 200 is the point at which tapered portion 215 andcylindrical portion 203 transition into one another. As shown on FIG.2A, threaded portion 202 is deployed on tapered portion 215, at end 201of fastener 200. FIG. 2A illustrates threaded portion 202 deployed onlypartially over tapered portion 215. It will be appreciated, however, inother embodiments (not illustrated), threaded portion 202 may deployedover the entire length of tapered portion 215.

The taper profile on tapered portion 215, whether constant or otherwise,is pre-selected. Moreover, as will be described in more detail furtheron, the dimensional characteristics of the threads on threaded portion202 (such as diameter, pitch, depth and profile) are also pre-selected.

FIG. 2B illustrates bone specimen 220 in cross-section, showing near andfar cortices 225, 226 surrounding interior portion 230. It will beappreciated that interior portion 230 may comprise cancellous boneand/or fatty bone marrow. Bone specimen 220 as illustrated on FIG. 2Bhas been prepared to receive fastener 200 from FIG. 2A. Morespecifically, opening 235 has been pre-configured to receive fastener200. Circular entry hole 240 extends through near cortex 225 intointerior portion 230 of bone specimen 220. In the embodiment illustratedin FIG. 2B, the diameter of entry hole 240 is pre-selected to be D_(C),substantially the same diameter as the diameter of cylindrical portion203 on fastener 200 (as shown on FIG. 2A). However, as will be discussedin more detail with reference to FIG. 2C, additional embodiments mayprovide entry hole 240 in bone specimen 220 with diameters or profilesother than D_(C). The invention is not limited with respect to anyparticular diameter or profile of entry hole 240 in bone specimen 220.

With continuing reference to FIG. 2B, opening 235 further providestapered female threaded section 245 provided in far cortex 226, whereinthe threads on tapered female threaded section 245 are pre-selected tomate with threaded portion 202 on fastener 200 (as shown on FIG. 2A). Ina preferred embodiment, such thread mating advantageously includesmating of thread taper, pitch, depth and profile.

FIG. 2C shows fastener 200 (as illustrated in isolation on FIG. 2A)received into opening 235 in bone specimen 220 (as illustrated inisolation on FIG. 2B). FIG. 2C shows threaded portion 202 on fastener200 operably secured into threaded section 245 in opening 235. As notedimmediately above in the discussion of FIG. 2B, the threads on threadedportion 202 on fastener 200 are pre-selected to mate with the threads onthreaded section 245 in opening 235 in bone specimen 220. In this way,as fastener 200 on FIG. 2C is threadably engaged into opening 235, thetaper on the interface between threaded portion 202 and threaded section245 causes threads of increasingly larger diameter on threaded portion202 to engage threaded section 245 in bone specimen 220. As a result, asfastener 200 on FIG. 2C is threadably engaged into opening 235, radialpreload is imparted into the area of far cortex 226 immediatelysurrounding threaded section 245.

It will be appreciated that the inventive material disclosed herein isindependent of the manufacturing method by which threaded section 245 isprovided in opening 235. Threaded section 245 may be provided by anysuitable technique, such as, without limitation, self-drilling,pre-drilling and reaming, or step-drilling, with subsequent tapping asrequired.

Since the thread characteristics of threaded portion 202 and threadedsection 245 are pre-selected, a pre-desired amount of radial preload canbe imparted into far cortex 226. Conventional methods may be used tomeasure, calculate or predict the amount of radial preload beingimparted. For example, conventional algorithms or tables may be used todetermine how many turns of fastener 200 will impart a pre-desiredamount of radial preload. Alternatively, a conventional torque wrenchmay be used to tighten fastener 200, where pre-calibration (such as viaa look-up table) will dictate how much torque imparted on fastener 200is equivalent to a pre-determined desired amount of radial preload.

Alternatively, embodiments of fastener 200 may be provided to limit themaximum radial preload available to be imparted by further tighteningfastener 200 once threaded portion 202 and threaded section 245 arethreadably engaged. FIGS. 4A to 4C illustrate exemplary embodiments ofstructure configured to limit the maximum radial preload available to beimparted in such “mating tapered thread” configurations. It will beappreciated, however, that the inventive material disclosed herein isnot limited to any particular structure provided to limit radial preloadonce threaded portion 202 and threaded section 245 are threadablyengaged, if indeed any such structure to limit radial preload isprovided at all.

FIG. 4A shows, as also illustrated on FIG. 2A, fastener 200 providingthreaded portion 202 on tapered portion 215. In the embodimentillustrated in FIG. 4A, however, unthreaded portion 203 on fastener 200is identified, as well as discontinuity 204. It will be appreciated thatunthreaded portion 203 transitions into threaded portion 202 atdiscontinuity 204. FIG. 4A also identifies D_(D) on fastener 200, thediameter along tapered portion 215 at which discontinuity 204 isprovided.

FIG. 4A also shows tapered female threaded section 445 disposed toreceive threaded portion 202 on fastener 200. It will be appreciatedthat in the embodiments for limiting “mating tapered thread” radialpreload described with reference to FIGS. 4A to 4C, bone preparationssuch as threaded section 445 on FIG. 4A, disposed to receive threadedportion 202 on fastener 200, may be located in either near or farcortices of a bone specimen. The location of bone preparations such asthreaded section 445 on FIG. 4A is independent of the inventive materialdisclosed herein describing structure configured to limit the maximumradial preload available to be imparted in “mating tapered thread”configurations.

Referring again to FIG. 4A, and analogous to threaded portion 202 andthreaded section 245 as described above with reference to FIG. 2C, thethreads on threaded portion 202 on FIG. 4A are pre-selected to mate withthe threads on threaded section 445. Again analogous to threaded portion202 and threaded section 245 on FIG. 2C, since the threadcharacteristics of threaded portion 202 on FIG. 4A and threaded section445 are pre-selected, a pre-desired amount of radial preload can beimparted into the bone material surrounding threaded section 445 bythreadably engaging threaded portion 202 on threaded section 445 andthen further tightening. However, in the embodiment illustrated on FIG.4A, threaded section 445 provides diameter D_(T) at its widest opening(i.e., its entryway), and D_(D) on fastener 200 and D_(T) on threadedsection 445 are pre-selected so that D_(D) is no more than about 0.2 mmgreater than D_(T).

Now, with continued reference to FIG. 4A, it will be understood that asthreaded portion 202 is threadably engaged on threaded section 445 andfurther tightened, discontinuity 204 on fastener 200 will act as a“stop” to prevent further tightening of fastener 200 into threadedsection 445 once discontinuity 204 reaches the first thread on threadedsection 445. Since D_(D) on fastener 200 and D_(T) on threaded section445 are pre-selected so that D_(D) is no more than about 0.2 mm greaterthan D_(T), the maximum radial preload available to be imparted into thebone material surrounding threaded section 445 is no more than about 0.2mm.

It will be readily appreciated that, within the scope of the embodimentillustrated in FIG. 4A, D_(D) and D_(T) can also be pre-selected to havedifferent relative values, so as to limit the maximum radial preloadavailable to be imparted to amounts other than about 0.2 mm. However, astaught by Biliouris et at, D_(D) and D_(T) in the embodiment illustratedin FIG. 4A should preferably be configured to limit the maximum radialpreload available to about 0.2 mm in order to avoid cause micro-crackingand other damage to the cortical bone.

Further to FIG. 4A and the accompanying disclosure immediately above,FIG. 4B illustrates another embodiment of structure configured to limitthe maximum radial preload available to be imparted in “mating taperedthread” configurations. Fastener 400 in FIG. 4B is similar to fastener200 in FIGS. 2A and 4A, except that discontinuity 404 separates taperedthreaded portion 402 and cylindrical unthreaded portion 403. Threadedportion 402 on fastener 400 on FIG. 4B is, however, inventively the samein all respects as threaded portion 202 on FIG. 4A, with D_(D)identified as the diameter at discontinuity 404. In the embodiment ofFIG. 4B, however, D_(D) on threaded portion 402 is the same as thecylindrical diameter of unthreaded portion 403. It will be appreciatedthat in some applications, the embodiment illustrated in FIG. 4B may bemore convenient to manufacture, by cutting threaded portion 402 onto acylindrical profile with a pre-selected diameter of D_(D). In suchembodiments, it will be understood to be important to establishdiscontinuity 404 at the transition from unthreaded portion 403 tothreaded portion 402 via, for example, a thread profile transitioneffective to prevent further tightening of fastener 400 once thethread(s) at D_(D) on threaded portion 402 engage the thread(s) at D_(T)on threaded section 445 (as illustrated on FIG. 4A).

Further to FIGS. 4A and 4B, and the corresponding accompanyingdisclosure immediately above, FIG. 4C illustrates yet another embodimentof structure configured to limit the maximum radial preload available tobe imparted in “mating tapered thread” configurations. Fastener 470 inFIG. 4C is similar to fastener 200 in FIGS. 2A and 4A, except thatdiscontinuity 474 separates tapered threaded portion 472 and taperedunthreaded portion 473 at a point of larger diameter than is desired tobe pre-selected as D_(D). Stop 480 is provided instead on threadedportion 472 at pre-selected diameter D_(D). It will be appreciated thatwith the exception of stop 480, rather than discontinuity 474, beingprovided on threaded portion 472 to define D_(D), threaded portion 472on fastener 470 on FIG. 4C is, however, inventively the same in allrespects as threaded portion 202 on FIG. 4A. In this way, if fastener470 on FIG. 4C is threadably engaged on threaded section 445 on FIG. 4Aand further tightened, stop 480 will prevent further tightening offastener 470 into threaded section 445 once stop 480 reaches the firstthread on threaded section 445.

Stop 480 on fastener 470 is illustrated on FIG. 4C as a washer. Theinventive material disclosed herein is not limited in this regard,however. Stop 480 may be any structure on threaded portion 472 suitableto define D_(D) and to prevent further tightening past D_(D). Otherexamples might include a lip or the head of a screw or bolt.

With reference back now to FIG. 2C, it will be appreciated that whenfastener 200 is threadably engaged into opening 235 so as to impartradial preload in far cortex 226, care should be taken so as not toexceed about 0.2 mm of radial preload. As taught by Biliouris et al.,and discussed above, amounts of radial preload in excess of about 0.2 mmhave been shown to cause micro-cracking and other damage to the corticalbone. Consideration should be given to deploying structure on fastener200, such as, without limitation, structure from among the examplessuggested above with reference to FIGS. 4A to 4C, in order to limit themaximum radial preload available to be imparted by “mating taperedthread” configurations to about 0.2 mm. It will be appreciated, however,that any such structure to limit radial preload imparted by matingtapered threads is optional within the scope of the inventive materialdisclosed herein.

As shown on FIGS. 2A and 2B, and discussed briefly above, theillustrated embodiment in FIGS. 2A, 2B and 2C provide cylindricalportion 203 on fastener 200 with substantially the same diameter D_(C)as entry hole 240 on bone specimen 220. It will be readily appreciated,however, that in other embodiments (not illustrated) entry hole 240 maybe provided with a diameter less than D_(C). In this way, when fastener200 is introduced into opening 235 on bone specimen 220, and cylindricalportion 203 is pressed into entry hole 240, radial preload may beimparted into near cortex 225.

Again, care should be exercised when providing an entry hole 240 on bonespecimen 220 with diameter less than D_(C), in order not to impart morethan about 0.2 mm of radial preload when cylindrical portion 203 ispressed into entry hole 240. As taught by Biliouris et al., and asdiscussed above, radial preloads exceeding about 0.2 mm may damage nearcortex 225. Indeed, it will be appreciated that further embodiments offastener 200 (not illustrated) may provide an arrangement on near cortex225 similar to those illustrated on FIG. 1C, in which a limited amountof radial preload is imparted when cylindrical portion 203 is pressedinto a reduced-diameter entry hole 240. In these embodiments, taperedportion 215 (as shown on FIG. 2A) may be of a pre-selected length sothat, when introduced into opening 235 on bone specimen 220 (as shown onFIG. 2C), reference points 205 are now located in near cortex bone 225,or outside of bone specimen 220 completely. The diameter of entry hole240 in bone specimen 220 in these embodiments is pre-selected to be nosmaller than about 0.2 mm less than D_(C). By analogy to FIG. 1C and itsaccompanying disclosure above, such alternative (but not illustrated)embodiments will allow radial preload in near cortex 225 to be limitedto no more than about 0.2 mm.

With further reference to FIGS. 2B and 2C, it will be appreciated that,although not illustrated, other embodiments may provide opening 235 witha profile through near cortex 225 that is shaped other than acylindrical entry hole such as item 240, as illustrated. For example,the profile of opening 235 through near cortex 225 could be tapered. Insuch alternative embodiments, it will be understood that D_(C), asillustrated on FIG. 2B, will be provided as a predetermined minimumentry diameter constricting the profile of opening 235 through nearcortex 225.

Turning now to FIGS. 3A, 3B and 3C, which should be viewed together, athird aspect of the present disclosure is illustrated, in which anorthopedic fastener 300 is configured to limit and/or control radialpreloads exerted in both near cortex 325 and far cortex 326 as fastener300 is operably secured into bone specimen 320. For general reference,subject to the detailed disclosure that follows, fastener 300, as shownoperably secured in bone specimen 320 on FIG. 3C, limits and/or controlsradial preloads in both near and far cortices 325 and 326 infunctionally the same way as fastener 200 limits and/or controls radialpreloads in far cortex 226 on FIG. 2C, as described above.

With reference first to FIG. 3A, orthopedic fastener 300 providestapered portion 315 on one end 301 of fastener 300. Although theinvention is not limited in this regard, FIG. 3A shows tapered portion315 comprising substantially the entire length of fastener 300. Otherembodiments (not illustrated) may provide fastener 300 with additionallength via, for example, a cylindrical portion provided distal from end301 and tapered portion 315, and/or a cylindrical portion appended toend 301.

FIG. 3A further illustrates threaded portion 302 provided on taperedportion 315, at end 301 of fastener 300. Threaded portion 302 isdeployed over threaded portion length 316. FIG. 3A illustrates threadedportion length 316 deployed substantially over the entire length oftapered portion 315. It will be appreciated, however, in otherembodiments (not illustrated, but discussed below with reference to FIG.3C), threaded portion length 316 may deployed at end 301 of fastener 300but over only part of tapered portion 315.

The taper profile on tapered portion 315, whether constant or otherwise,is pre-selected. Further, threaded portion length 316 is alsopre-selected. Moreover, as will be described in more detail further on,the dimensional characteristics of the threads on threaded portion 302(such as diameter, pitch, depth and profile) are also pre-selected.

FIG. 3B illustrates bone specimen 320 in cross-section, showing near andfar cortices 325, 326 surrounding interior portion 330. It will beappreciated that interior portion 330 may comprise cancellous boneand/or fatty bone marrow. Bone specimen 320 as illustrated on FIG. 3Bhas been prepared to receive fastener 300 from FIG. 3A. Morespecifically, opening 335 has been pre-configured to receive fastener300. Opening 335 begins at entry 340, and provides tapered femalethreaded section 345 through near cortex 325, interior portion 330, andfar cortex 326. The threads on tapered female threaded section 345 arepre-selected to mate with threaded portion 302 on fastener 300 (as shownon FIG. 3A). In a preferred embodiment, such thread matingadvantageously includes mating of thread taper, pitch, depth andprofile.

Analogous to the disclosure above describing FIGS. 2B and 2C, it will beappreciated on FIGS. 3B and 3C that the inventive material disclosedherein is independent of the manufacturing method by which femalethreaded section 345 is provided in bone specimen 320. Threaded section345 may be provided by any suitable technique, such as, withoutlimitation, self-drilling, pre-drilling and reaming, or step-drilling,with subsequent tapping as required.

FIG. 3C shows fastener 300 (as illustrated in isolation on FIG. 3A)received into opening 335 in bone specimen 320 (as illustrated inisolation on FIG. 3B). FIG. 3C shows threaded portion 302 on fastener300 operably secured into threaded section 345 in opening 335. Note thatnotation of opening 335 has been omitted from FIG. 3C for clarity, butmay be identified via reference to FIG. 3B.

It will be appreciated from FIG. 3C that threaded portion length 316 onfastener 300 (as shown on FIG. 3A) is pre-selected to enable threadedportion 302 on fastener 300 (as shown on FIG. 3C) to engage threadedsection 345 on bone specimen 320 in both near cortex 325 and far cortex326. Moreover, as noted immediately above in the discussion of FIG. 3B,the threads on threaded portion 302 on fastener 300 are pre-selected tomate with the threads on threaded section 345 in opening 335 in bonespecimen 320. In this way, as fastener 300 on FIG. 3C is threadablyengaged into opening 335, the taper on the interface between threadedportion 302 and threaded section 345 causes threads of increasinglylarger diameter on threaded portion 302 to engage threaded section 345in bone specimen 320.

As a result, as fastener 300 on FIG. 3C is threadably engaged intoopening 335, radial preload is imparted into the areas of near cortex325, interior portion 330 and far cortex 326 immediately surroundingthreaded section 345. As noted above, this mechanism to impart radialpreload is functionally the same as the manner in which fastener 200limits and/or controls radial preloads in far cortex 226 on FIG. 2C. Itwill be understood, however, that although fastener 300 on FIG. 3C iscapable of imparting radial preload into interior portion 330 as well asnear and far cortices 325 and 326, any radial preload operably retainedin interior portion 330 is not of inventive significance to thisdisclosure. While such radial preload retained in interior portion 330,if any, may be of some practical assistance preventing loosening offastener 300 under operating loads, the inventive focus of thisdisclosure is on radial preload imparted by fastener 300 on near and farcortices 325 and 326.

With further reference to FIG. 3C, since the thread characteristics ofthreaded portion 302 and threaded section 345 are pre-selected, apre-desired amount of radial preload can be imparted into near cortex325 and far cortex 326. Conventional methods may be used to measure,calculate or predict the amount of radial preload being imparted. Forexample, conventional algorithms or tables may be used to determine howmany turns of fastener 300 will impart a pre-desired amount of radialpreload. Alternatively, a conventional torque wrench may be used totighten fastener 300, where pre-calibration (such as via a look-uptable) will dictate how much torque imparted on fastener 300 isequivalent to a pre-determined desired amount of radial preload.

It will be recalled from earlier disclosure herein that FIGS. 4A to 4Cillustrate exemplary embodiments of structure configured to limit themaximum radial preload available to be imparted in “mating taperedthread” configurations, such as have just been described with referenceto FIGS. 3A to 3C. It will be understood that any of the embodimentsillustrated, disclosed or suggested above with respect to FIGS. 4A to 4Cmay be deployed on fastener 300 as illustrated and described withreference to FIGS. 3A to 3C. It will be further understood that any ofthe embodiments illustrated, disclosed or suggested above with respectto FIGS. 4A to 4C may be deployed on fastener 300 so as to limit themaximum radial preload available to be imparted in the near cortex only,in the far cortex only, or concurrently in both cortices. It will alsobe appreciated, however, that the inventive material disclosed herein isnot limited to any particular structure provided to limit the maximumradial preload available to be imparted by fastener 300, if indeed anysuch structure to limit radial preload is provided at all in eithercortex.

With reference back now to FIG. 3C, it will be appreciated that whenfastener 300 is threadably engaged into opening 335 so as to impartradial preload in near cortex 325 and far cortex 326, care should betaken so as not to exceed about 0.2 mm of radial preload. As taught byBiliouris et al., and discussed above, amounts of radial preload inexcess of about 0.2 mm have been shown to cause micro-cracking and otherdamage to the cortical bone. Consideration should be given to deployingstructure on fastener 300, such as, without limitation, structure fromamong the examples suggested above with reference to FIGS. 4A to 4C, inorder to limit the maximum radial preload available to be imparted by“mating tapered thread” configurations to about 0.2 mm. As noted above,however, it will be appreciated that any such structure to limit radialpreload imparted by mating tapered threads is optional within the scopeof the inventive material disclosed herein

It was mentioned above in reference to FIG. 3A that, although notillustrated, additional length could be provided to fastener 300 via,for example, a cylindrical portion provided distal from end 301 and/or acylindrical portion appended to end 301. With reference now to FIG. 3C,it will be appreciated that such additional length is not of inventivesignificance to this disclosure. Additional length, distal from end 301and threaded portion length 316, may be provided to fastener 300 by anystructural means, so long as threaded portion length 316 (as shown onFIG. 3A) is pre-selected to permit threaded portion 302 on fastener 300(as shown on FIG. 3C) to engage threaded section 345 in both near cortex325 and far cortex 326.

Throughout this disclosure so far, embodiments of fasteners have beendescribed with structural features that may be pre-selected to matchcorresponding features of bone openings into which they may be received.It will be further appreciated, however, that the converse is truewithout departing from the spirit and scope of the inventive materialdisclosed herein. More specifically, with particular reference to FIGS.1C, 2C and 3C, it will be appreciated that fasteners according to thisdisclosure may be manufactured as “rated” to be received incorresponding bone hole preparations. It will be understood that when afastener is rated to be received in a corresponding bone holepreparation, the fastener is manufactured with accompanying disclosurefrom the manufacturer regarding specific characteristics of acorresponding bone hole preparation that will engage the inventivefunctionality of the fastener.

For example, in the embodiments disclosed in FIGS. 1A, 1B and 1C,fastener 100 may be manufactured as rated to be received in acorresponding bone hole preparation comprising: (1) a diameter D_(O) ofentry hole 140 that is pre-selected to be smaller than D_(F) ofcylindrical portion 103 on fastener 100, but no more than about 0.2 mmsmaller than D_(F); and (2) an overall thickness of bone specimen 120that is pre-selected to be in a range of thicknesses that permit taperedportion 115 on fastener 100 to contact entry hole 140 when threadedportion 102 is threadably engaged in far cortex 126.

Similarly, by way of further example in the embodiments disclosed inFIGS. 2A, 2B and 2C, fastener 200 may be manufactured as rated to bereceived in a corresponding bone hole preparation comprising: (1) adiameter D_(C) of entry hole 240 that is pre-selected to besubstantially the same as D_(C), the diameter of cylindrical portion 203on fastener 200, but in no circumstances more than about 0.2 mm smallerthan D_(C); and (2) a threaded section 245 provided in far cortex 226that is pre-selected to mate with threaded portion 202 on fastener 200for taper, thread pitch, thread depth and thread profile.

Similarly, by way of further example in the embodiments disclosed nFIGS. 3A, 3B and 3C, fastener 300 may be manufactured as rated to bereceived in a corresponding bone hole preparation comprising: (1) athreaded section 345 provided in near cortex 325 through to far cortex326 that is pre-selected to mate with threaded portion 302 on fastener300 for taper, thread pitch, thread depth and thread profile; and (2) anoverall thickness of bone specimen 320 that is pre-selected to be in arange of thicknesses that permit threaded portion length 316 on fastener300 to engage threaded section 345 in both near cortex 325 and farcortex 326.

Embodiments of the invention have been described herein with referenceto orthopedic fixation pins. It will be appreciated, however, that theinvention is not limited in this regard, and that it may be embodied onany orthopedic fastener, such as, without limitation, pedicle screws,orthopedic nails, bone screws, hip screws, or even dental implants.

Those of ordinary skill in this art will further appreciate that theinvention is not limited to any particular size of orthopedic fastener.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalternations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

I claim:
 1. A method for limiting radial preload in securing anorthopedic fastener into a corresponding bone specimen, the methodcomprising the steps of: (A) selecting the corresponding bone specimento have an overall outer thickness from near cortex to far cortex in apredetermined range of outer bone thicknesses; and (B) securing afastener into the bone specimen, the fastener comprising means forimparting radial preload into the near cortex as the fastener isoperably secured into the far cortex through an opening in the nearcortex, the fastener further comprising means for limiting said radialpreload to a maximum of about 0.2 mm.
 2. The method of claim 1, in whichthe fastener further comprises means for imparting radial preload intothe far cortex as the fastener is operably secured into the far cortexthrough an opening in the near cortex, and in which the means forimparting radial preload into the far cortex comprises a plurality oftapered male threads having a pre-selected thread profile.
 3. The methodof claim 2, in which the fastener further comprises means for limitingthe maximum radial preload available to be imparted into the far cortex.4. The orthopedic fastener of claim 2, further comprising means forlimiting the maximum radial preload available to be imparted into thefar cortex to about 0.2mm.
 5. A method for limiting radial preload insecuring an orthopedic fastener into a corresponding bone specimen, themethod comprising the steps of: (A) selecting and preparing thecorresponding bone specimen to have (1) an overall outer thickness fromnear cortex to far cortex in a predetermined range of outer bonethicknesses, and (2) a predetermined bone hole preparation including anentry portion in the near cortex, the entry portion having apredetermined minimum entry diameter; (B) providing a fastener, thefastener including a shank having adjacent threaded and unthreadedportions, the unthreaded portion providing a tapered diameter portionlocated at a preselected location on the shank, the tapered diameterportion transitioning from a large taper diameter to a small taperdiameter, the small taper diameter located proximate to the threadedportion, the large taper diameter located distal from the threadedportion, the small taper diameter not greater than the predeterminedminimum entry diameter in the bone hole preparation, the large taperdiameter not greater than about 0.2 mm more than said predeterminedminimum entry diameter; and (C) limiting radial preload in the nearcortex to not more than about 0.2 mm by securing the threaded portion onthe fastener into the far cortex of the bone specimen so that thetapered diameter portion of the fastener contacts the entry portion inthe near cortex.
 6. The method of claim 5, in which step (A) includespreparing the bone specimen to further comprise an entry portion in thenear cortex including a cylindrical entry hole of predetermined holediameter, and in which (1) the small taper diameter on the fastener isnot greater than said predetermined hole diameter, and (2) the largetaper diameter on the fastener is not greater than about 0.2 mm morethan said predetermined hole diameter.
 7. The method of claim 5, inwhich step (A) includes preparing the bone specimen to further comprisea female threaded section provided in the far cortex, threads on thefemale threaded section configured to threadably engage with threads onthe threaded portion of the fastener.
 8. The method of claim 5, in whichthe threaded portion on the fastener includes self-tapping threadssuitable for anchoring into cortical bone.
 9. The method of claim 5, inwhich step (A) includes preparing the bone specimen to further include atapered female threaded section in the far cortex, threads on thetapered female threaded section having a pre-selected thread profile,and in which the threaded portion on the fastener further comprises: aplurality of male tapered threads, the male tapered threads configuredto mate, according to the pre-selected thread profile, withcorresponding threads on the tapered female threaded section; thepre-selected thread profile having a predetermined thread geometry, saidthread geometry predetermined so that, when the male tapered threads onthe fastener are fully engaged in the tapered female threaded section onthe bone hole preparation, a predetermined further tightening of thefastener imparts a corresponding predetermined radial preload on bonetissue surrounding the tapered female threaded section.
 10. The methodof claim 9, in which: the plurality of male tapered threads includes athread stop, the thread stop located such that the thread stop enablesno more than a maximum male thread diameter D_(D) on the plurality ofmale tapered threads to engage the tapered female threaded section inthe far cortex; the tapered female threaded section provides a maximumfemale thread diameter D_(T); and D_(D) and D_(T) are pre-selected sothat (D_(D)-D_(T)) provides a maximum radial preload available to beimparted on bone tissue surrounding the tapered female threaded section.11. The method of claim 10, in which D_(D) and D_(T) are pre-selected sothat (D_(D)-D_(T)) is about 0.2 mm.
 12. The orthopedic fastener of claim10, in which the thread stop is selected from the group consisting of:(a) a thread discontinuity; (b) a washer; (c) a lip; and (d) a fastenerhead.
 13. A method for limiting radial preload in securing an orthopedicfastener into a corresponding bone specimen, the method comprising thesteps of: (A) selecting and preparing the corresponding bone specimen tohave (1) an overall outer thickness from near cortex to far cortex in apredetermined range of outer bone thicknesses, and (2) a predeterminedbone hole preparation including a cylindrical entry hole in the nearcortex of predetermined entry hole diameter; (B) providing a fastener,the fastener including a shank having adjacent threaded and unthreadedportions, the unthreaded portion providing a tapered diameter portionlocated at a preselected location on the shank, the tapered diameterportion transitioning from a large taper diameter to a small taperdiameter, the small taper diameter located proximate to the threadedportion, the large taper diameter located distal from the threadedportion, the small taper diameter not greater than the predeterminedentry hole diameter in the bone hole preparation, the large taperdiameter not greater than about 0.2 mm more than the predetermined entryhole diameter; and (C) limiting radial preload in the near cortex to notmore than about 0.2 mm by securing the fastener into the far cortex ofthe bone specimen so that the tapered diameter portion of the fastenercontacts the cylindrical entry hole in the near cortex.
 14. The methodof claim 13, in which step (A) includes preparing the bone specimen tofurther comprise a female threaded section provided in the far cortex,threads on the female threaded section configured to threadably engagewith threads on the threaded portion of the fastener.
 15. The method ofclaim 13, in which the threaded portion on the fastener includesself-tapping threads suitable for anchoring into cortical bone.
 16. Themethod of claim 13, in which step (A) includes preparing the bonespecimen to further include a tapered female threaded section in the farcortex, threads on the tapered female threaded section having apre-selected thread profile, and in which the threaded portion on thefastener further comprises: a plurality of male tapered threads, themale tapered threads configured to mate, according to the pre-selectedthread profile, with corresponding threads on the tapered femalethreaded section; the pre-selected thread profile having a predeterminedthread geometry, said thread geometry predetermined so that, when themale tapered threads on the fastener are fully engaged in the taperedfemale threaded section on the bone hole preparation, a predeterminedfurther tightening of the fastener imparts a corresponding predeterminedradial preload on bone tissue surrounding the tapered female threadedsection.
 17. The method of claim 16, in which: the plurality of maletapered threads includes a thread stop, the thread stop located suchthat the thread stop enables no more than a maximum male thread diameterD_(D) on the plurality of male tapered threads to engage the taperedfemale threaded section in the far cortex; the tapered female threadedsection provides a maximum female thread diameter D_(T); and D_(D) andD_(T) are pre-selected so that (D_(D)-D_(T)) provides a maximum radialpreload available to be imparted on bone tissue surrounding the taperedfemale threaded section.
 18. The method of claim 17, in which D_(D) andD_(T) are pre-selected so that (D_(D)-D_(T)) is about 0.2 mm.
 19. Theorthopedic fastener of claim 17, in which the thread stop is selectedfrom the group consisting of: (a) a thread discontinuity; (b) a washer;(c) a lip; and (d) a fastener head.